Production of purified strong wet-process phosphoric acid

ABSTRACT

The concentration of impurities in wet-process phosphoric acid limits the usefulness of the acid for the production of fertilizers, feed-grade phosphate and detergents. The aluminum, magnesium, and fluoride impurities are removed with the calcium sulfate hemihydrate filter cake by hydrolyzing and recycling the off-gas scrubber solutions in the presence of a ferric iron catalyst. The aluminum and magnesium are reduced to less than 0.1 percent in the acid product by the precipitation of (Ca,Mg)Al 2  F 8 .2H 2  O, which is acid insoluble and suitable for storage with the byproduct calcium sulfate. Ferric iron also serves as a catalyst for the simultaneous precipitation of (Na,K) 2  SiF 6 . Subsequent additions of potassium as, for example, in the teachings of Gilbert (U.S. Pat. No. 3,338,675), can then be applied to obtain a purified phosphoric acid product suitable for most commercial applications. The added potassium serves two purposes in the 40 to 55 percent P 2  O 5  acid since ferric iron is reduced to 0.25 percent Fe 2  O 3  by the precipitation of Fe 3  KH 14  (PO 4 ) 8 .4H 2  O and the SiF 6   =  is reduced to about 0.2 percent by the precipitation of K 2  SiF 6 . This subsequent precipitate contains about 50 percent P 2  O 5  and is as effective as superphosphate [Ca(H 2  PO 4 ) 2 .H 2  O] in supporting plant growth.



